On a C3, there is a short, heavy gauge ground cable that goes from the engine mount on the frame to the starter. If you look at the AIM this cable attaches to engine block (not the starter) where the starter brace attaches (same bolt). Presumably there is continuity between the block and starter through the mounting bolts.

I am using a GM mini starter and there is no ground stud on it. Do you think the ground connection though the block to starter mount is OK?

I usually chase/clean the threads in the block, scrape any paint from the starter mount surface on the block and give it a skim coat of dielectric grease. Assuming a good block to chassis to battery connection, this should be more than adequate. This is how we have a 10.5:1 BBC (with plenty of initial ignition advance) set up at our shop and it cranks over quite quick